Thermal Subsidence Phase Research Articles

Early Mesozoic rift basins of eastern North America and their gravity anomalies: The role of detachments during extension

Two end‐member models of extension involving detachments have been developed. One model incorporates a fault that soles at mid‐crustal level overlying a broad region of pure shear in the lower crust. The second, referred to as the simple shear model, includes a detachment continuing through the entire crust and terminating in a region of concentrated extension in the lower crust. Both models predict basins with no localized thermal effect. With the inclusion of flexural isostasy, both models predict footwall uplift whose amplitude and wavelength are controlled by the detachment geometry and the lithospheric strength. A gravity anomaly over the hanging wall block distinguishes the simple shear model from the intracrustal detachment model. The early Mesozoic basins of the eastern North America, believed to have formed as the result of the normal‐slip reactivation of a Paleozoic thrust system as the Atlantic opened, are associated with distinctive hanging wall gravity highs. These gravity highs, the basin geometry, the lack of a thermal subsidence phase in the rift basins, and the presence of a highly extended and heated region to the east, suggest that the simple shear model may be applied. The simple shear model fits the outer hanging wall anomaly and permits a region of lower crustal extension to be mapped. These basins contain an abundance of basalt flows and diabase sills despite the lack of evidence for regional heating or thermal subsidence, implying that a source, external to the basin, must exist for this magmatic material. The detachment fault may facilitate the movement of the molten mafic material into the rift basins from an offshore region of greater heating and extension. The addition of 2 km of mafic material along the model detachment accounts for the observed inner gravity high and the lack of a negative gravity anomaly across these basins. The match of this modified simple shear model to the observed gravity suggests that the region of greater extension seaward of the hinge zone is the source for the widespread dikes and sills within the basins and the coastal plain and helps explain the geochemical homogeneity of these intrusives and extrusives along 2000 km of the eastern seaboard.

The Structure of the West Orkney Basin, northern Scotland

The West Orkney Basin developed in Devonian times, as the western part of the Orcadian intermontane basin. It has been studied using commercial speculative seismic reflection data and the MOIST deep seismic data. The NW edge of the West Orkney Basin is formed by listric faults which are also strongly arcuate in plan, while the SE part is composed of straight domino-type faults which formed parallel to earlier (Caledonian) layering in the basement. Fault restoration and balancing suggest that initial extension in the basin occurred on low-angle reactivated Caledonian thrust faults. Steeper breaching faults cut the low-angle set, forming planar (domino-type) faults in the centre of the basin but listric faults at the NW margin. The maximum extension is about 45% in the basin centre, most of this being taken up on the later breaching fault system. This extension decreases to the SW, where fault tips occur on-shore, but some may transfer to fault systems in the Minches. The faults apparently decouple at a depth of 18-20 km and the extension suggests an initial post-Caledonian crustal thickness of up to 40 km. However, the sedimentary thickness is an average of only 3 km in the basin centre, much less than would be expected had the lithosphere thinned homogeneously, and there is no evidence of a thermal subsidence phase to the basin. This suggests that the extension shown by the West Orkney Basin was transferred to lower lithospheric levels to the east along the deep decoupling zone. The Devonian sediments on-land show facies changes and periods of uplift and erosion which may be related to extension during basin development. They also show a phase of pre-Late-Permian tectonic inversion where the beds are locally folded and thrusted, probably related to the Hercynian events further south. The West Orkney Basin is capped by Mesozoic sediments and was probably reactivated during Mesozoic basin development in the Minches and Moray Firth. The shape of the faults, their orientation and decoupling levels are strongly controlled by the earlier Caledonian structure, in particular by the layering and crustal anisotropy developed along and above the Moine thrust. The West Orkney Basin with its 20 km deep decoupling level formed by extension of Caledonian thickened crust. It is notable that the major basin-bounding faults to the NW, the Outer Isles and Flannan faults, which developed where the crust was thinner and hence less ductile at depth, decoupled at much deeper structural levels, at the Moho or below.

Geochemical and Geochronological Signatures in Australian Proterozoic Igneous Rocks

Summary Most igneous activity in Australian Proterozoic domains is related to major orogenic events, or else to crustal extensional events in which there is an earlier volcanogenic rift phase overlain by a non-volcanic thermal subsidence phase. The igneous suites are either predominantly mafic, felsic or bimodal, and there are no major suites of intermediate igneous rocks. With time, the amount of igneous material decreases volumetrically in the Proterozoic sedimentary sequences and there are also distinct compositional changes. Depleted tholeiites are more common in the early Proterozoic sequences, whereas enriched tholeiites and alkaline rocks are dominant after 1800 Ma. Felsic igneous rocks are rare in the earliest Proterozoic sequences, but dominate in the period 1880-1400 Ma. They are also compositionally time controlled, and certain major magmatic felsic suites at c . 1860, 1790, 1740, 1670, 1500 Ma have distinct chemical parameters which are common to most domains. All the major suites of felsic igneous rocks show enrichment in K, Rb, Th, U, La, and Ce relative to Phanerozoic analogues. The felsic rocks are predominantly I-types, derived from Proterozoic infracrustal rather than Archaean precursors; rare S-types also appear to have Proterozoic sources. Our preferred model has the felsic igneous rocks derived from underplated sources which are progressively accreted to the base of the crust, as a result of a series of extensional episodes. There are no direct compositional equivalents of modern subduction-related igneous sequences suggesting that subduction as it is known today was not a dominant process in the production of the igneous suites. There are also no recognizable ophiolites, ocean-floor crust, or other evidence of a Wilson cycle.

Subsidence in Late Paleozoic basins in the northern Appalachians

During the interval between continental collision in the Devonian and continental breakup in the Triassic the northern Appalachians became the site of a wide plate boundary zone of dominantly right‐lateral strike slip. As is typical of intracontinental transforms, tectonism was both diachronous and rapidly variable along strike through regimes of ‘pure’ strike slip, transpressional deformation, and rapid subsidence of extensional basins. Up to 9 km of mainly nonmarine, clastic sediments accumulated in these local depocenters, which subsided episodically in two stages: (1) an initial phase of stretching and thinning of the lithosphere, when subsidence was rapid, fault controlled, and often accompanied by volcanism and (2) a subsequent phase of gradual thermal subsidence, during which the depositional basins expanded to bury the earlier border faults and progressively younger sedimentary units onlapped basement. The largest depocenter, the Magdalen Basin, opened as a pull‐apart between strike slip faults in Newfoundland and New Brunswick from late Devonian to early Carboniferous. Subsequent thermal subsidence affected a large area during medial and late Carboniferous, a phenomenon that is well recorded to the north and west, where no later tectonism occurred. In areas to the south and east of the basin, strike slip on other faults continued into the time of thermal subsidence, introducing complications such as localized transpressional deformation and rapid subsidence in smaller pull‐aparts.